The present invention relates to multi-cylinder Diesel engines of the type employing variably actuated valves.
In American patent U.S. Pat. No. 6,237,551, the Applicant has already proposed an engine of this type including:
two inlet valves and two exhaust valves for each cylinder, each equipped with respective elastic means of return that push the valve towards its closed position, for controlling the respective inlet and exhaust ports,
at least one camshaft for operating the inlet and exhaust valves of the engine""s cylinders via respective valve lifters, each inlet valve and the two exhaust valves being controlled by a respective cam of the said camshaft,
in which each of the said valve lifters commands the respective inlet or exhaust valve against the action of the said elastic means of return via the interposition of hydraulic means including a pressurized fluid chamber.
the pressurized fluid chamber associated with each inlet valve or with the two exhaust valves being suitable for connection via a solenoid valve to a discharge channel, for the purposes of decoupling the valve from its respective valve lifter and provoking rapid closure of the valve under the effect of the elastic means of return,
electronic means of control for controlling each solenoid valve for varying the time and travel of the respective inlet or exhaust valve according to one or more of the engine""s operating parameters,
in which each cam on the engine camshaft has a profile such that it tends to provoke the opening of the respective inlet valve or respective exhaust valves that it controls, not only during the conventional opening phase of the engine""s normal operating cycle, but also in certain additional phases of the cycle,
in which the said electronic means of control are capable of provoking the opening of each solenoid valve to maintain the respective inlet valve or the respective exhaust valves closed during the above-mentioned conventional phase and/or during one or more of the said additional phases in which the respective cam would tend to provoke the opening of the valve, in consequence of which the engine can be made to selectively run according to different modes of operation controlled by the said solenoid valves, and
in which the profile of the cam controlling the exhaust valves is such as to cause an additional opening phase of the exhaust valves substantially during the final part of the induction phase, thereby realizing an operating cycle of the so-called xe2x80x9cpost-chargingxe2x80x9d type, where, due to excess pressure in the inlet port, the opening of the exhaust valves during the final part of the induction phase causes fresh air to first flow directly from the inlet port to the exhaust port, while successively, following the increase in pressure in the exhaust port, part of the air returns from the exhaust port, entering the cylinder by exploiting the excess pressure in the exhaust port, thereby improving cylinder replenishment.
The object of the present invention is that of further perfecting the previously proposed engine for the purposes of achieving a series of advantages with regards to reducing harmful exhaust emissions and/or overcoming problems of cold starting or creating the so-called xe2x80x9cblue smokexe2x80x9d in the xe2x80x9cwarm-upxe2x80x9d phase after a cold start, and/or achieving improved performance and/or consumption reductions.
In order to achieve this objective, the subject of the invention is an internal combustion engine possessing all of the above indicated characteristics and also characterized by the fact that the cam controlling each inlet valve is shaped such that it provokes the opening of the respective inlet valve during the engine""s normal exhaust phase to accomplish exhaust gas recirculation (EGR) inside the engine, due to the fact that during the normal exhaust phase part of the exhaust gas passes from the cylinder into the inlet port, and then returns to the cylinder during the next induction phase, while part of the exhaust gas that previously passed into the exhaust port returns into the cylinder during this induction phase due to the said additional opening of the exhaust valve, in consequence of which the exhaust gas charges that return to the cylinder participate in the combustion on the next engine cycle.
Thanks to internal EGR, it is possible to achieve substantial reductions in consumption and emissions at low revs and loads when cold. As can be seen, to realize both the xe2x80x9cpost-chargingxe2x80x9d cycle and internal EGR, an additional opening of the exhaust valves is needed during the induction phase. Nevertheless, maximum efficiency in the two cases is achieved with a different law and lift timing for the exhaust valves. Thanks to the use of variably actuated valves, it is possible to employ a cam with a predetermined geometry to achieve both objectives, since the aforesaid electronic means of control that intervene can realize, for a given cam geometry, different lift geometries for the exhaust valves.
In traditional engines, internal EGR can only be realized in a limited measure, as there would otherwise be an excessive reduction in the xe2x80x9cswirlxe2x80x9d of the air charge introduced into the cylinder due to the introduction of a mass of burnt gases with an angular motion that is null, or low or in the opposite direction. According to another characteristic of the invention, in order to significantly reduce emissions via an increase in internal EGR tolerability, the end sections of the two inlet ports associated with each cylinder are shaped such that one channels air into the cylinder in an almost tangential direction, while the other, having a spiral shape, generates a rotating vortex around an axis that is substantially parallel to the axis of the cylinder, the said electronic means of control being capable of controlling the two inlet valves associated with these ports in a differentiated manner and so modulate the level of xe2x80x9cswirlxe2x80x9d within the cylinder. In fact, the first inlet port, with the tangential outlet, is suitable for generating significant xe2x80x9cswirlxe2x80x9d from the first stages of inlet valve opening, while the second port has the function of xe2x80x9creplenishmentxe2x80x9d and only generates xe2x80x9cswirlxe2x80x9d in synergy with the first. In this way, it is possible to choke air induction whilst maintaining high swirl, thereby avoiding the risks of stalling that are generated due to excessive EGR. Keeping the first port open and choking valve lift on the second minimises losses due to the surge effect, which have a negative effect on consumption.
Thanks to control over the effective compression ration, the engine according to the invention can also exploit, in the same manner as the known engine already proposed, the possibility of designing an engine with a relatively low geometric compression ratio, in the order of 17:1 for example or even lower. The electronic means of control can thus be set up to close the inlet valve after bottom dead centre at maximum revolutions and loads and to instead advance the closure of the inlet valve to bottom dead centre during starting. In this way, when starting, all of the cylinder""s internal volume is exploited to avoid the risk of misfire and producing xe2x80x9cblue smokexe2x80x9d due to low pressure and temperature, because all of the engine""s geometric compression ratio is exploited, whilst at maximum revolutions and loads a law for valve lift similar to the conventional one is used.
According to another characteristic of the invention, the electronic means of control are set up to advance the closure of the inlet valves and/or to advance the opening of the exhaust valve on cold starts in order to reduce the flow of air through the engine and, in consequence, for a given amount of heat transferred to the exhaust gas, to increase exhaust temperature, to obtain the same result. Thanks to these characteristics, it is possible to obtain an increase in exhaust gas temperature during warm-up that is useful for activating exhaust gas post-treatment systems (catalysers and traps). In certain known engines, this result is achieved via a butterfly valve inserted in the inlet port, which has the drawback however of limited dynamic response.
Naturally, the fact that the engine according to the invention exploits an EGR system within the engine does not exclude the possibility of also using external EGR. In general, internal EGR (hot) is not as efficient as external EGR (cooled) in reducing nitrogen oxides. In any case, internal EGR (hot) can be used for reducing nitrogen oxides during the first phases of engine warm-up where the amount of external EGR cannot be maximised due to its low temperatures, which results in excessive emission levels of carbon and hydrocarbon oxides.
Another important advantage of the engine according to the invention, deriving from the possibility of using internal EGR, is that of obtaining an HCCI (Homogeneous Charge Compression Ignition) type of combustion. In fact, the variable valve actuation system can accurately control high internal EGR doses and dilute the charge to render it almost stoichiometric, simultaneously controlling its temperature by mixing with external EGR (cold). This is extremely important because the temperature of the charge influences the ignition delay caused by the high rarefaction of the mixture and, thanks to the high concentration of active radicals present in hot EGR, it can accelerate the speed of combustion.
The system also allows improved charge homogenisation and stratification. The control of the inlet and exhaust valves in a differentiated manner can be adjusted according to the engine""s state of operation and allow stratification of air and internal EGR in a controllable manner. From the viewpoint of controlling self-ignition and combustion, the combination of this stratification of gases with the possibility of introducing fuel in small packets (multiple injection) allows homogenisation and/or stratification of the fuel/air/residual gases right from the very first phases of induction. In addition, the injection of a small amount (pilot) of fuel during the last phases of compression allows the charge to be locally enriched and guarantees its ignition and combustion.
The use of an oxygen sensor opportunely positioned on the engine exhaust allows continuous correction in the actuation of the valves and/or the introduction of fuel for correcting the effective mixture strength of each cylinder on a cycle-by-cycle basic.
Finally, the cycle-by-cycle control of air and internal EGR provided by the variable valve actuation system permits transition from HCCI combustion to conventional Diesel combustion without any vehicle driveability problems.
In the engine switch-off phase, inlet and/or exhaust valve lifts are modulated to minimise compression pressure inside the cylinder and, in consequence, torque oscillations on the engine shaft. This strategy significantly reduces engine/vehicle shaking and substitutes the butterfly device, inserted in the inlet line, which is currently used for the same purposes.
In addition, the possibility of selectively closing both the inlet and exhaust valves of any cylinder, even while running, allows the other cylinders to operate with higher charges and thus in a more efficient manner in terms of fuel consumption (modularity).